The invention concerns a current source which allows rapid voltage fluctuations on its output without affecting the current being delivered. This characteristic of the source is due partly to its structure and partly to the fact that it comprises NPN transistors.
A current source is by definition a circuit which must supply a stable current to another electronic circuit. However, during operation, through changes of state, rapid fluctuations of current can occur in the second circuit, which affect the output of the current source.
If the current source has low impedance, it can supply the current required, but this low impedance produces a reaction which destabilizes the output current. If, on the other hand, the current source has high impedance, it is more stable but can not respond to rapid fluctuations.
The diagram of a current source according to known configurations is shown in FIG. 1. It is very simple and includes a current mirror formed by the transistors Q1 and Q2 and by the current source Q3: this source is regulated using a reference voltage which is produced at the terminals of a resistor R.sub.ref, and its temperature is controlled by the standard V.sub.BG and by the transistor Q.sub.ref. The transistor Q4 is mounted symmetrically to transistor Q3.
If R3=R4 and if the transistors Q3 and Q4 have the same geometry, they deliver the same currents, and in particular Q3 delivers a current equal to I.sub.ref. If on the other hand, transistor Q1 has a geometry "n" times greater than that of Q2, it delivers "n" times more current: for example, if n=5, the output current is six times greater than the reference current I.sub.ref (1.times.I.sub.ref across Q2+5.times.I.sub.ref across Q1).
This architecture has the advantage of being very simple, requiring few transistors and having low consumption. It is an improvement in the sense that the current mirror Q1+Q2, comprising NPN transistors, which amplifies the current, makes it possible to eliminate current gain fluctuations in transistor Q3, which is a PNP transistor.
However, in fast bipolar technology, PNP transistors generally have more gain disperston than NPN transistors.
In addition, the dynamic performances of PNP transistors such as Q3 and Q4 are very inferior to those of NPN transistors such as Q1 and Q2, because the stray capacitances of a PNP transistor are greater than those of an NPN transistor . In these conditions, a rapid fluctuation in the output current I.sub.S (or in the output voltage V.sub.S ) is not instantly transmitted to the base of the PNP Q3 because of its collector-base stray capacitance, and Q3 does not react quickly enough to correct this fluctuation.
Finally, the modulation of the collector current I.sub.C as a function of the collector-emitter voltage (known as the "Early voltage"), is very low ior a PNP transistor , which makes the output current I.sub.S dependent on the output voltage V.sub.S, thus causing static inaccuracy.